A pixel is not really a unit, and a px is unit in illustrator. Making pixel a unit is a bit problematic since its not a natural unit but rather a quantity. For one pixel in image is not the same thing as a pixel as a unit. This is important. A natural unit is fixed in relation to other narural units. We call natural units: units. Therefore a pixel is not a unit in the same sense it has no dimension.
Now the image has a quantity of pixels which in your case is 524x449 pixels. But this has no natural size it can be a kilometer wide or it can be one inch wide. That is just up to how big your image is set to be. You call pictre image pixel quantity the dimensions of a image.
Illustrator works on physical units because it simulates reality. The pixel unit (not to be confused with pixel quantity) which inside illustrator is defined as 1/72th of an inch. Which coincidentally is a point. Any other application may define pixels as whatever size they like (w3c has desfined svg pixel as 1/92th of an inch, therefore illustrator svgs arent standards compilant). Pixel as a unit is coerced not natural and you end up with several things you call a pixel, because ultimately its a quantity.
Now pixels in your image are quantities and pixels in illustrator are units. Two separate things, or in a page description language like illustrator you could have many things named a pixel that have different sizes.
When illustrator imports an image it makes the image same size as any resolution metadata of your image specifies. If your image has any resolution other than 72 ppi then pixels as units don't match the pixels of your image. Your image may have same amount of pixels as before but they dont match size of pixels as units.
Why do we have pixels as units... Well it was requested by people with little understanding of Metrology (art of units). Now there is only one way to do this correctly and never allow pixels as units. All other aporoaches will lead to problems for somebody eventually.